I have a question about the way a controllers radar works. How does the radar identify you on the radar scope? Like, is it essential that you have a transponder to even become an object on his radar scope? I know that the transponder sends data blocks that report altitude and stuff like that but if you were to turn off your transponder off, like they did in 9/11, would the controller still be able to see you on his scope? Thanks

The altitude will vary depending on the distance from the radar site, obstructions, etc. Radar picks up metal the best, but will show flocks of birds, precip, and sometimes even smoke from a large fire.

so, if you were in an airplane and you turned your transponder off, you then fly level or below a mountain, how would the controller know you from a car or tower on that mountain?

That's a lot of "ifs", but I'll give it a go. If you are that low in relation to a mountain it's possible there may not be any radar coverage so the controller would deny all service. If there is radar coverage and the controller has the a digitized radar the controller can "tag up" a primary target so it will show a callsign, type and even the altitude the pilot reports. If it's not digitized the controller would probably deny service. Another factor is cars driving on mountain roads move very slowly and erratically compared to the flight profile of most airplanes. And towers don't move at all.

Thanks for the information, guys. The reason I was asking was because the other day I was talking with a pilot friend of mine and he mentioned that without a transponder, the controller would not be able to see you at all. He said that a transponder was essential for them to see you AT ALL. He also said that he had never heard of a 'primary radar' before. So I looked it up and sort of proved him wrong about that but then he asked me about that scenario that I mentioned above.

I know that the scenario that I gave would be pretty unlikely to happen at all and had a lot of 'what ifs' but I was just using sort of an extreme

To keep it simple, most radar systems have Moving Target Indicator filters that remove returns that are not moving or moving too slow. Other than that it comes from distinguishing what is clutter/permanent echo from what is possibly an aircraft.

Thanks for the information, guys. The reason I was asking was because the other day I was talking with a pilot friend of mine and he mentioned that without a transponder, the controller would not be able to see you at all. He said that a transponder was essential for them to see you AT ALL. He also said that he had never heard of a 'primary radar' before. So I looked it up and sort of proved him wrong about that but then he asked me about that scenario that I mentioned above.

I know that the scenario that I gave would be pretty unlikely to happen at all and had a lot of 'what ifs' but I was just using sort of an extreme

An operational transponder is a requirement in certain airspace classes and controllers may deny service to a pop-up aircraft w/o a transponder depending on workload and radar performance. Also controllers don't like working planes without a transponder. But that is not the same thing of not being able to see a primary target.

So, I assume that the dots with numbers beside them are airplanes that have been identified on radar, the white lines are runways, and everything else shown is either mountains or other objects that the radar picked up?

The light blue circle contains and aircrafts Primary return with and SSR return overlaid, not necessarily identified; the to 4 digits of the label are the assisgned SSR code, below that is the Mode C readout and an arrow to indicate whether it's climbing or descending.

The green circle depicts a Primary return for an aircraft.

Yes the white lines indicate the extended centrelines, the background shading indicates different types of airsapce.

And the small, numerous orange dots are Primary returns which are clutter; terrain, weather, birds etc....

However as a previous poster explained; mountains and such are usually filtered out by use of MTI.

The primary target to the East may or may not have been identified at the time. There are a variety of methods for identification of PSR targets; but if the aircraft wasn't talking to ATC unit then it would not be identified.